US-South Korea Collaborative Research: Additive Manufacturing of Fatigue Resistant Materials

增材制造或3D打印提供了制造定制的复杂金属部件的能力，这些部件传统上无法用于各种应用。因此，工程设计和产品实现的范式转变正在发生，许多行业，如生物医学和航空航天，都准备从中受益。一些例子包括：(1)现场快速制造具有患者和损伤特定设计的金属骨植入物，以及(2)在远程位置（例如外层空间）制造替代部件。然而，目前的增材制造方法制造的金属部件往往具有孔隙率和各向异性，这些特征通常不利于部件的强度和抗疲劳性。这些部件不能在承重应用中放心使用。该奖项支持通过增材制造生产抗疲劳金属部件的科学研究。本研究的目标是：(1)建立微观组织特性（晶粒取向和形貌）、孔隙度分布（尺寸、形状和位置）和工艺参数（激光功率、扫描速度、hatch间距和层向）之间的关系；(2)了解增材制造材料的显微组织性能（晶粒取向）和孔隙率对其抗多轴疲劳性能的影响。为了实现第一个目标，将开发连续尺度的热物理模型，并将其用于将微观结构特性和孔隙度分布与工艺参数联系起来。这些模型将经过实验验证。Ti-6Al-4V样品将在各种工艺参数组合下使用激光增材制造制造。层的方向将在0º-90º之间改变，而扫描速度、舱口间距和激光功率将在现有知识（例如已发表的实验数据）规定的范围内变化。将使用x射线断层扫描（孔隙度的大小、形状和位置）以及光学和扫描电子显微镜（晶粒取向和形貌）来测量制备样品的微观结构特性和孔隙度分布。为了实现第二个目标，基于临界平面方法的多轴微结构敏感疲劳模型将被开发并通过实验验证。对预制试件进行多轴疲劳试验，采用同相和异相区分加载路径，行使不同的临界加载面。将对试样的断口表面进行断口学，以确定引起裂纹的孔的位置、大小和形状。裂纹复制技术将用于寻找疲劳微裂纹在临界加载平面上的取向，并确定各向异性微观结构对疲劳行为的影响。通过与韩国工业技术研究院的合作，生成的模型将在其他增材制造方法和材料上进行测试。

Additive manufacturing, or 3D printing, offers the ability to fabricate customized, complex metallic parts traditionally unobtainable for a variety of applications. A paradigm shift in engineering design and product realization is thus occurring, and many industries, such as biomedical and aerospace, are poised to benefit. Some examples include (1) on-site, rapid fabrication of metallic bone implants with patient and injury-specific designs, and (2) fabrication of replacement parts in remote locations (e.g. outer space). Nonetheless, metallic parts made by current additive manufacturing methods tend to have porosity and anisotropy, features typically detrimental to part strength and fatigue resistance. Such parts cannot be used with confidence in load-bearing applications. This award supports scientific investigation that can potentially enable production of fatigue resistant metallic parts by additive manufacturing. The objectives of this research are (1) to establish relationships between microstructure properties (grain orientation, and morphology), porosity distribution (size, shape, and location), and process parameters (laser power, scanning speed, hatch spacing, and layer orientation); and (2) to understand effects of microstructure properties (grain orientation) and porosity of additively-manufactured materials on their multi-axial fatigue resistance. To achieve the first objective, continuum-scale thermophysical models will be developed and used to relate microstructure properties and porosity distribution to process parameters. These models will be experimentally validated. Ti-6Al-4V specimens will be fabricated using laser-based additive manufacturing under various process parameter combinations. The layer orientation will be altered between 0º-90º, while scanning speed, hatch spacing, and laser power will be varied within ranges prescribed from existing knowledge (e.g. published experimental data). Microstructure properties and porosity distribution of fabricated specimens will be measured using X-ray tomography (size, shape, and location of porosity), as well as optical and scanning electron microscopy (grain orientation, and morphology). To achieve the second objective, multi-axial microstructure-sensitive fatigue models based on critical plane approaches will be developed and validated by experiments. Multi-axial fatigue experiments will be conducted on fabricated specimens, using in-phase and out-of-phase discriminating load paths to exercise different critical loading planes. Fractography on the fracture surface of specimens will be performed to determine location, size, and shape of the pore(s) responsible for initiating cracks. Crack replication techniques will be employed to find the orientation of fatigue micro-cracks with respect to critical loading plane and to determine effects of anisotropic microstructure on fatigue behavior. Through the collaboration with the Korea Institute of Industrial Technology, the generated models will be tested on other additive manufacturing methods and materials.